Lean Ground Turkey: Ground turkey makes an excellent substitution for ground hamburger. You can exchange it pound per pound with hamburger, or you can do what we often do and use half turkey, half hamburger in our most common recipes. Substitute turkey for beef in your favorite tacos, burritos, lasagnas, pastas, and eggrolls. Be sure to buy lean turkey or ground turkey breast because all other ground turkey (or chicken) contains skin and other unmentioned parts. The taste should be comparable to beef, but if you find you don't like it as much, experiment with spices. I have found that adding marjoram and savory to ground turkey make it taste great!
Mushrooms: Mushrooms are an excellent alternative or addition to almost any meat filled meal. I love to add mushroom to omelets and lasagna, with or without meat. They are also fantastic on pizza! Next time you are running low on meat or just want to reduce the amount of meat you are using, try substituting mushrooms. For a smooth transition, cut the mushrooms to the same size your meat would have been.
Ground Pork: Ground pork is inexpensive and can be a lower fat alternative to ground beef if you buy the lower fat versions. The same rules apply to pork as turkey...add a few spices if you want to enhance the flavor.
Beans: Kidney beans and pinto beans are a great addition or replacement for meat in many cases, and not just in chili. You can also use them in tacos and burritos, lasagna (really any veggie works great in lasagna, experiment with them), and pastas.
Use less: One trick I use is to decrease the amount of meat we eat, is to just use less. I generally buy hamburger in the 3 lb packages and separate them into 1/3 - 1/2 lb packages and freeze them individually. In a recipe that calls for 1lb of ground beef, I either use the smaller 1/3lb package or use the smaller package and use one of the other options listed above.